At this point there hasn't been any new FCC filing and those are usually discovered months before any new device announcement. And the announcements are made well before anything is released.
And there's no promise Kobo's next device will be another 6" device.
For my money, the only features the Clara HD is missing are:
- Waterproofing
- Flush screen
- Page turn buttons
- More storage
From that list, I would guess Kobo's next six inch reader will have maybe #1, maybe #2. #3 would be nice but is unlikely. Probably the same for #4*
Only you can decide if any of those are worth waiting for.
At this point I'd say get a Clara, or take a step up and get a Libra.
*and you can add storage to a Clara if you really want to, though for me 8GB is plenty.